Add FLAG_always_promote_young_mc that always promotes young objects
during a Full GC when enabled. This flag guarantees that the young gen
and the sweeping remembered set are empty after a full GC.

This CL also makes use of the fact that the sweeping remembered set is
empty and only invalidates an object when there were old-to-new slots
recorded on its page.

Bug: chromium:1014943
Change-Id: Idfb13dfbe76bad5ec8b485a60bebc30531aec649
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1863201
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64348}